My AP Biology ThoughtsUnit 8 Episode #21

Welcome to My AP Biology Thoughts podcast, my name is Shriya and I am your host for episode #21 called “Plant Behavior (phototropism, gravitropism, thigmotropism)”. Today we will be discussing the definitions of all of those concepts as well as a few examples to go along with them. Then, we will connect all of that to the overarching topic of evolution. Hope you enjoy!.

Segment 1: Introduction to Plant Behavior (phototropism, gravitropism, thigmotropism)

• We will be discussing the topic of plant behavior which is how plants respond to changes in their environments
• Like all organisms, they detect and respond to stimuli in their environments, but they cannot physically move to get away from danger since they are rooted to the soil
• Instead, a plants primary means of response is to change how it is growing based on a number of factors 
• Their responses are normally hormone based because they do not have nervous systems to control them
• They can have responses to both internal and external stimuli, however in this episode we will mainly be focusing on their external responses to stimuli
• Plant roots always grow downwards because they have specialized cells that respond to gravity which is an example of a tropism 
• A tropism is turning towards or away from a stimulus in the environment
• The first type of tropism is called phototropism which means the plant is growing towards a light source 
• This response is controlled by a plant growth hormone called auxin
• Auxin stimulates cells on the dark side of a plant to grow longer which provokes the plant to bend towards the light 
• The next type of tropism is called gravitropism, or geotropism where “geo” means the Earth and tropism refers to turning
• Gravitropism is the growth of a plant’s organ or change in the direction of its growth in response to gravity
• Since plant cells are able to sense gravity, if a root is not growing towards the center of the earth, the cells become aware of that and change it
• Auxin is also stimulated either at the root caps or the shoots to allow cell elongation
• Finally, there is also thigmotropism which is the response to touch 
• The stimulating factor is generally a hard surface that can change the direction of the plant’s growth or the growth of one of its organs
• Thigmotropism can be in the form of...